We did this on another board and I haven't laughed so hard in a long time. Here's a couple of mine-
One weekend a dump truck was parked right across the street from our house, I'm about 8 years old. Well, we decided to check out the gas tank, now that I'm older and hopefully smarter, I have to assume it was deisel. Anyway, we decided the only way to find out if there was gas in the tank was to throw a lit match in. Holy crap! The flame that came out of that thing was amazing....a 10ft torch! Put the cap on and it went out, pretty cool. Yep, we did it a couple of more times before my friend's Dad tore us a new one.
The other one involves one of those tractors I mentioned above. Back then we found out they didn't need a key to start the engine. There was ths little switch on the dash that would power things up and a button to start the engine. As luck would have it, we found them and fired one of those puppies up. 3 - 8 year old kids driving a front loader around a huge construction site got us into a ton of trouble.
I can't even imagine what I'd do to my kids if they did half the crap I did as a kid!